The need for heat transfer between gas streams arises in many industrial processes, for energy saving and heat recovery purposes. For example, heat recovery is practiced with flue gases from combustion processes for reasons of economy, so that the hot gases leaving the processes can preheat incoming gases. This is undertaken in many metallurgical processes such as smelting and blast furnace operations, where spent, discharge gas issues from the furnace at high temperature, and incoming, reactant gas is at a lower temperature, but must be hot enough to maintain the reaction temperature.
Methods currently employed for heat transfer between gas streams, at high temperatures, are inefficient. In one method, checkerwork brick recuperators are used, which are alternately heated and cooled by the discharge gas and the inlet gas respectively. Present heat recovery practices using recuperators are inefficient on account of the low heat transfer rates between the gas streams and the brick apparatus, the large volume necessary for the recuperator apparatus, the high capital cost and high maintenance cost of the recuperator.
A special case of the need for heat transfer between gases arises in the case of processes for gasification of solid carbonaceous fuel deposits such as oil shales, tar sands and coal. The gaseous fuels which are produced from coal are, basically, mixtures of carbon monoxide and hydrogen along with hydrocarbon and small amounts of carbon dioxide. Gasification of, for example, coal requires the reacting of the coal at very high temperatures with steam, so as to produce a fuel-rich gas comprising a mixture of, predominantly, carbon monoxide and hydrogen. This reaction is endothermic. To achieve the necessary high reaction temperatures (e.g. above 700.degree. C.) and supply heat to the endothermic fuel gas producing reaction, an exothermic reaction is conducted, namely combustion of a small amount of the coal with oxygen. Heat from the exothermic reaction is then transferred to the endothermic, fuel gas producing reaction.
Some coal gasification processes currently in use involve intermittent feed of air, followed by water vapour, to the coal bed. The air causes combustion of some of the coal and raises the temperature. The subsequent feeding of water vapour produces fuel gas but at the same time causes cooling of the coal. Then air is fed through again, to raise the temperature ready for a subsequent injection of water vapour. In other coal gasification processes, mixtures of oxygen and water vapour at high temperatures are fed into the coal, so that the exothermic and endothermic reactions may proceed together. In such a mixed feed process, however, one has to use oxygen rather than air, or the fuel gas produced will be diluted with nitrogen. This adds to the expense of the process. The intermittent, cyclic process can use air, since no fuel gas is being produced when air is fed in, and the nitrogen can therefore be bled off and kept away from the fuel gas.